A pandemic is large scale epidemic caused by an infectious disease that has spread to a whole continent or the whole world.  The global economy and travel trends of the 21st centruy has increase the risk of a pandemic taking hold.

The largest threat today from an pandemic is the Plague which is spread by the Yersinia pestis bacteria.  The most common forms of the Plague are Bubonic plague, Pneumonic plague and Septicemic plague all of which have caused major pandemics in human history.